Topic 16 HL - Chemical Kinetics

Question 1

rate of reaction =

Answer 1

change in concentration / time

Question 2

reactions may occur by more than one step but …

Answer 2

the slowest step determines the rate of reaction (rate determining step RDS)

Question 3

the molecularity of an elementary step is …

Answer 3

the number of reactant particles taking part in that step

Question 4

the order of reaction can be either …

Answer 4

integer or fractional nature

Question 5

the order of reaction can describe …

Answer 5

with respect to a reactant, the number of particles taking part in the rate determining step

Question 6

rate equations can only be determined…

Answer 6

experimentally because the orders can only be deduced empirically

Question 7

the value of the rate constant (k) is affected by …

Answer 7

temperature and its units are determined from the overall order of the reaction

Question 8

catalysts …

Answer 8

alter a reaction mechanism, introducing a step with lower activation energy

Question 9

overall reaction order =

Answer 9

m + n
the orders with respect to each reactant, which convey how sensitive the rate of reaction is to change in the concentrations or A and B

Question 10

a catalyst is a substance that …

Answer 10

increases the rate of a chemical reaction but is not consumed in the reaction itself. it provides an alternative pathway for the reaction and lowers the activation energy

Question 11

the contact process

Answer 11

1) S(s) + O2(g) –> SO2(g)
2) SO2(g) + O2(g) 2SO3(g)
the catalyst used is vanadium (V) oxide V2O5(s) which is a heterogenous catalyst
3) SO3(g) + H2SO4(l) –> H2S2O7(l). reacts with concentrated sulphuric acid to produce oleum
4) H2S2O7(l) + H2O(l) –> 2H2SO4(aq) sulphuric acid

Question 12

why is the oleum formed first?

Answer 12

the direct reaction between sulphur trioxide and water is too vigorous

Question 13

individual steps in a reaction mechanism are called …

Answer 13

elementary steps / elementary reaction / elementary process

Question 14

elementary steps are classified by their …

Answer 14

molecularity, which represents the number of molecules or atoms involved as reactants in the elementary step

Question 15

unimolecular

Answer 15

single molecule involved in an elementary step

Question 16

bimolecular

Answer 16

two molecules or atoms involved in collision in an elementary step

Question 17

termolecular

Answer 17

three molecules or atoms involved in collision in an elementary step

Question 18

each elementary step has its own…

Answer 18

rate constant, k, and its own activation energy Ea

Question 19

reaction: NO2(g) + CO2(g) –> NO(g) + CO2(g)

Answer 19

step 1) NO2(g) + NO2(g) –> NO(g) + NO3(g) step 1 is bimolecular. cross out intermediates (NO2 and NO3)
step 2) NO3(g) + CO(g) –> NO2(g) + CO2(g) step 2 is bimolecular. cross out intermediates (NO3 and NO2

Question 20

why is NO3 an intermediate?

Answer 20

it is formed in step 1 and then consumed subsequently in step 2

Question 21

how do you determine the slow step?

Answer 21

the rate of the overall reaction is equal to the rate of the slow step

Question 22

what is the experimental rate of reaction for the NO2 and CO2 reaction?

Answer 22

for any temperature under 498K, the experimental equation for the reaction is:
rate = k(NO2)^2

Question 23

the Arrhenius equations uses …

Answer 23

temperature dependence of the rate constant to determine the activation energy

Question 24

what must be fulfilled for a chemical reaction to occur between two reactant particles?

Answer 24

1) the two particles must collide with each other, that is there must be physical contact
2) the colliding particles must have correct mutual orientation
3) the colliding particles must have sufficient kinetic energy to initiate the reaction itself